The specialists are all incredibly well balanced, and their effectiveness is largely going to be determined by each person's play style. Some folks are aggressive, and like the King. Some are sneaky, and like the Navi Pirate. Others are diplomatic and defensive, and prefer Tycoon and Minister of Energy. Some of you seem to enjoy using the Admiral, but I would almost never retire a Navigator to get one, but that's just my play style. The specialists are all just different blokes for different folks.
I was wary of the King at first as well, then I tried using him. King is amazing. Small empire or Big, I love getting a King whenever I can. The obvious tradeoff is the loss of shields, but if you're on the offensive, you don't need no stinking shields. But the real tradeoff with the King is that he upgrades from Hypnotist, which is usually a marginal specialist. But the game changer is the Second King. There are no drawbacks whatsoever (other than needing two Hippies). By the time I get Three Kings, the game is usually over in a couple days and I can't really take advantage.
The Tycoon is awesome as well, but it's biggest tradeoff is that you have to retire your Smuggler. I almost never want to do that, because the Smuggler is so good. Smuggler is the one specialist where my game is significantly dampened by not getting one, and my opponents are significantly boosted by having one. However, even still, it's not a game changer. I'm playing one 10-player game where I inexplicably did not get a Smuggler until day 8 of the game, but I am still likely going to win -- mostly through a combination of good diplomacy (as RoadKiehl says), but also because I got a King super early!
It's also worth noting that through diplomacy, you can trade specialists with your neighbors if you don't like what you were given. One time I traded a Smuggler (the best specialist in the game, by far) for a Hypnotist (arguably one of the worst) and I got a second King from that and cruised to victory.